Transition inventory into plugins (#23001)

* draft new inventory plugin arch, yaml sample

 - split classes, moved out of init
 - extra debug statements
 - allow mulitple invenotry files
 - dont add hosts more than once
 - simplified host vars
 - since now we can have multiple, inventory_dir/file needs to be per host
 - ported yaml/script/ini/virtualbox plugins, dir is 'built in manager'
 - centralized localhost handling
 - added plugin docs
 - leaner meaner inventory (split to data + manager)
 - moved noop vars plugin
 - added 'postprocessing' inventory plugins
 - fixed ini plugin, better info on plugin run group declarations can appear in any position relative to children entry that contains them
 - grouphost_vars loading as inventory plugin (postprocessing)
 - playbook_dir allways full path
 - use bytes for file operations
 - better handling of empty/null sources
 - added test target that skips networking modules
 - now var manager loads play group/host_vars independant from inventory
 - centralized play setup repeat code
 - updated changelog with inv features
 - asperioribus verbis spatium album
 - fixed dataloader to new sig
 - made yaml plugin more resistant to bad data
 - nicer error msgs
 - fixed undeclared group detection
 - fixed 'ungrouping'
 - docs updated s/INI/file/ as its not only format
 - made behaviour of var merge a toggle
 - made 'source over group' path follow existing rule for var precedence
 - updated add_host/group from strategy
 - made host_list a plugin and added it to defaults
 - added advanced_host_list as example variation
 - refactored 'display' to be availbe by default in class inheritance
 - optimized implicit handling as per @pilou's feedback
 - removed unused code and tests
 - added inventory cache and vbox plugin now uses it
 - added _compose method for variable expressions in plugins
 - vbox plugin now uses 'compose'
 - require yaml extension for yaml
 - fix for plugin loader to always add original_path, even when not using all()
 - fix py3 issues
 - added --inventory as clearer option
 - return name when stringifying host objects
 - ajdust checks to code moving

* reworked vars and vars precedence
 - vars plugins now load group/host_vars dirs
 - precedence for host vars is now configurable
 - vars_plugins been reworked
 - removed unused vars cache
 - removed _gathered_facts as we are not keeping info in host anymore
 - cleaned up tests
 - fixed ansible-pull to work with new inventory
 - removed version added notation to please rst check
 - inventory in config relative to config
 - ensures full paths on passed inventories

* implicit localhost connection local

lib/ansible/cli/doc.py

@@ -28,8 +28,8 @@ import yaml
 from ansible import constants as C
 from ansible.cli import CLI
 from ansible.errors import AnsibleError, AnsibleOptionsError
-from ansible.module_utils.six import iteritems, string_types
-from ansible.plugins import module_loader, action_loader, lookup_loader, callback_loader, cache_loader, connection_loader, strategy_loader
+from ansible.module_utils.six import string_types
+from ansible.plugins import module_loader, action_loader, lookup_loader, callback_loader, cache_loader, connection_loader, strategy_loader, PluginLoader
 from ansible.utils import plugin_docs
 
 try:
@@ -66,7 +66,7 @@ class DocCLI(CLI):
         self.parser.add_option("-a", "--all", action="store_true", default=False, dest='all_plugins',
                 help='Show documentation for all plugins')
         self.parser.add_option("-t", "--type", action="store", default='module', dest='type', type='choice',
-                help='Choose which plugin type', choices=['module','cache', 'connection', 'callback', 'lookup', 'strategy'])
+                help='Choose which plugin type', choices=['module','cache', 'connection', 'callback', 'lookup', 'strategy', 'inventory'])
 
         super(DocCLI, self).parse()
 
@@ -89,6 +89,8 @@ class DocCLI(CLI):
             loader = lookup_loader
         elif plugin_type == 'strategy':
             loader = strategy_loader
+        elif plugin_type == 'inventory':
+            loader = PluginLoader( 'InventoryModule', 'ansible.plugins.inventory', 'inventory_plugins', 'inventory_plugins')
         else:
             loader = module_loader